It is conventional practice to manufacture concrete masonry units, such as concrete block or brick, utilizing portland cement in combination with aggregate. Prior practice has mixed one part of cement with ten parts aggregate, conventionally fine and coarse aggregate, usually in the form of sand and gravel or crushed stone. Small amounts of other materials can be added to promote or retard the reactions or to give desired properties to the cement. Portland cement is essentially a calcium-alumina-magnesia-silicate composition. Addition of water to a dry mixture of portland cement results in a series of crystalization reactions by virtue of which the concrete attains the strength necessary for use in building products. Due to the complexity of the various chemical reactions, much investigation has been given both to reaction conditions and reactants, to determine their effect on cured products. Such research has influenced the establishment of uniform quality standards for cement, as set forth by the American Society for Testing Materials (ASTM).
Among many additives utilized in the preparation of concrete and concrete block products are such materials as sulfates, as retarders, chlorides as accelerators, and pozzolans. These pozzolans materials in themselves have no cementitious or binding quality, but react with lime in the presence of water to form cementitious materials. Pozzolans include such materials as fly ash, silica flour, and silica-containing substances such as powdered brick, burned shale, and some slags. Suitable fly ash compositions for preparation of cement block include any of those fly ash compositions set forth by ASTM C 618, "Standard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for use as a Mineral Additive in Portland Cement Concrete". Generally produced from the burning of pulverized bituminous coal, and collected in large quantities by electrostatic precipitation, fly ash has been suggested as a suitable fine aggregate for concrete. For example, Nelles, U.S. Pat. No. 2.250,107 discloses the use of fly ash having an approximate chemical composition of about 43% silica, 25% alumina, 15% iron oxide, 2% calcium oxide, and 1% magnesia, as an aggregate for concrete. Rule, U.S. Pat. No. 2,527,766, suggests the preparation of building materials from fly ash by the use of small percentages of portland cement in combination with up to 85% fly ash. In addition, Bainton, U.S. Pat. No. 3,953,222, teaches a method for making concrete products, in which pozzolans are added to the concrete (or substituted for some of the portland cement therein) after acidulation of the pozzolan material with a strong mineral acid. Bainton teaches the preparation of cement block from compositions comprising 80% aggregate, 10% cement, and 10% pozzolan (fly ash).
In addition to the above, the prior art also teaches the combination of aggregate, cement, and lime kiln dust. Urschel, U.S. Pat. No. 4,026,716, teaches a cement composition, suitable for preparation of concrete block, incorporating a precipitate comprising CaO-MgO from the gaseous effluent from a kiln burning dolomitic limestone. The patentee indicates that from 5 to 15 weight percent of this lime kiln dust may be used, in combination with portland cement, limestone aggregate, and manufactured fine aggregate (obtained from the operation of a dolomitic limestone quarry). The manufactured fine aggregate, referred to in the trade as "manufactured sand", contains about 30% by weight calcium, calculated as CaO, and about 22 percent magnesium, calculated as MgO.
Nicholson, in U.S. Pat. No. 4,038,095, teaches a mixture consisting essentially of fly ash, lime stack dust, and aggregate, which through pozzolanic reactions produce a hard, strong, durable mass capable of supporting surfacing. Specifically, the patent teaches preparation of a pavement base utilizing from 71 to 85% aggregate, from 10 to 14% fly ash, and from 5 to 15% lime stack dust. The lime stack dust, from the precipitator of a limestone kiln, contains a mixture of raw kiln feed, partly calcined material, and finely divided material. Typically lime stack dust has a chemical composition of CaO, MgO, S, and CO.sub.2. When mixtures made in accordance with the invention set forth by Nicholson are mixed with water to produce a pozzolanic reaction, the mixtures meet or exceed the ASTM specifications for fly ash and other pozzolans for use with lime.
However, certain problems are inherent in the use of lime or lime stack dust. Slaking or hydration of the lime must be closely supervised to ensure that hydration is thorough, and in the manufacture of concrete block structures, difficulties are often encountered, due to the lack of cementitious quality of the lime, which has no significant silicate content.